Fresnel reflection

[Guest]

Are all materials supposed to have a Fresnel reflection to be physically correct? The Fresnel IOR then obviously differs for each material. In addition to this, if I put a falloff map in the reflection slot and change its type to Fresnel, is this the same as putting a pure white colour in the reflection and then turning on Fresnel reflections?

Thanks

[Guest]

Are the reflection and refraction IOR's supposed to be the same?

[glorybound]

I have always thought an architectural IOR chart would be great, but I have never found one.

[Guest]

I see loads on the net, but they all give different values. It makes me think that it simply doesn't exist and there is no final value. Its just what looks good, and correct. I may be wrong and hopefully someone will explain

[MHolmstrom]

if I put a falloff map in the reflection slot and change its type to Fresnel, is this the same as putting a pure white colour in the reflection and then turning on Fresnel reflections?

Yes, it's the same. Using a falloff map can be a bit slower than using the built-in fresnel though.

As for physical correctness, fresnel reflections are a good approximation for dielectric materials (glass, plastic, water, etc.). Metals do not necessarily reflect light in a fresnel kind of way, even if a high IOR fresnel can still make it look good. And then there's materials like retroreflectors which can't be approximated with fresnel at all.
The (in terms of physical correctness) best way to recreate materials is to use BDRF-measurements from real materials. Unfortunately, V-Ray doesn't support this (IIRC, Vlado said it'd be too slow to render)

It's good to keep in mind that even a physically accurate solution is not necessarily the best looking one. So it's good to study these things, but there's no reason to waste time making everything as physically accurate as possible rather than just making it look good by eyeballing

[Guest]

Thanks for the reply, really useful. Does glass have any form of reflective glossiness? Or should this be kept at 1.0?

[MHolmstrom]

Does glass have any form of reflective glossiness? Or should this be kept at 1.0?

I'd keep it at 1.0, unless you're making ground glass. In real life it's probably something around 0.998, but the difference is so subtle that it'd just be a waste of render time.
Fingerprints, oily dirt and scratches can be simulated fast and easy with a bitmap in the specular glossiness slot.

[Guest]

Thanks again.

[bob-cat]

The material is more reflective if you use a falloff map set to "Fresnel" as apposed to enabling the Vray "Fresnel" checkbox.

I am not sure why and have always wanted to know.

Maybe different calculations, but if someone could elaborate on this that would be great.


Cheers

[plastic_]

With glass and metal the reflection falloff is more a property of the surface treatment, than the material.
The fresnel tables on the net are bogus IMO, at least for rendering.
It comes down to what looks right, there is no perfect formula for real world materials.

[Guest]

Some great tips here I also wanted to know if a highlight glossiness is added to a material does this effect the reflective glossiness? ie make it worse/incorrect? Or does it just add a fake highlight on top?

Also would it add to render time if highlight glossiness is used as well as reflective glossiness?

[Deflaminis]

I've been curious about this stuff too. Many sites I've seen put gold ranging anywhere from .47 to 41! whoa... How can they be so different? How can any of them be less than 1 in the first place? I don't remember all my physics but isn't that a vacuum at that point?

Do the reflective caustics use the same IOR as in the refraction tab or should we be putting a falloff with a custom IOR in there?

[simmsimaging]

Some great tips here I also wanted to know if a highlight glossiness is added to a material does this effect the reflective glossiness? ie make it worse/incorrect? Or does it just add a fake highlight on top?

Also would it add to render time if highlight glossiness is used as well as reflective glossiness?

Pretty sure it works the other way: reflective glossiness will affect your highlights, but highlight gloss is just for speculars created by non-realistic lights (point lights) and won't affect reflections at all.

/b

[Guest]

Ah ok cool, i agree there should be some sort of IOR table thats correct. Along with percentage values for reflection/refraction.